Use the improved kwargs_to_strings decorator

pygmt/src/timestamp.py

@@ -5,11 +5,12 @@
 
 from packaging.version import Version
 from pygmt.clib import Session, __gmt_version__
-from pygmt.helpers import build_arg_string, is_nonstr_iter
+from pygmt.helpers import build_arg_string, kwargs_to_strings
 
 __doctest_skip__ = ["timestamp"]
 
 
+@kwargs_to_strings(offset="sequence")
 def timestamp(
     self,
     text=None,
@@ -83,14 +84,11 @@ def timestamp(
         kwdict["U"] += f"{label}"
     kwdict["U"] += f"+j{justification}"
 
-    if is_nonstr_iter(offset):  # given a tuple
-        kwdict["U"] += "+o" + "/".join(f"{item}" for item in offset)
-    elif "/" not in offset and Version(__gmt_version__) <= Version("6.4.0"):
+    if "/" not in offset and Version(__gmt_version__) <= Version("6.4.0"):
         # Giving a single offset doesn't work in GMT <= 6.4.0.
         # See https://github.com/GenericMappingTools/gmt/issues/7107.
-        kwdict["U"] += f"+o{offset}/{offset}"
-    else:
-        kwdict["U"] += f"+o{offset}"
+        offset = f"{offset}/{offset}"
+    kwdict["U"] += f"+o{offset}"
 
     # The +t modifier was added in GMT 6.5.0.
     # See https://github.com/GenericMappingTools/gmt/pull/7127.

pygmt/src/which.py

@@ -6,13 +6,14 @@
     GMTTempFile,
     build_arg_string,
     fmt_docstring,
-    is_nonstr_iter,
+    kwargs_to_strings,
     use_alias,
 )
 
 
 @fmt_docstring
 @use_alias(G="download", V="verbose")
+@kwargs_to_strings(fname="sequence_space")
 def which(fname, **kwargs):
     r"""
     Find the full path to specified files.
@@ -62,9 +63,6 @@ def which(fname, **kwargs):
     FileNotFoundError
         If the file is not found.
     """
-    if is_nonstr_iter(fname):  # Got a list of files
-        fname = " ".join(fname)
-
     with GMTTempFile() as tmpfile:
         with Session() as lib:
             lib.call_module(